The spring steel used in the past for suspension springs, leaf springs, and so forth in automobiles, or in various types of industrial machinery and so on, was mainly JIS SUP11, SUP10, SUP9, SUP6, and steel equivalent to these, but the trend toward weight reduction in automobiles in recent years made it all the more important to reduce the weight of the springs themselves, which are suspension devices.
There has been a need for greater design stress to this end, and for the development of high-stress spring steel that can accommodate these higher stresses. Moreover, the need for higher hardness is particularly great with large-diameter suspension springs with a diameter of 30 mm or more and thick leaf springs with a thickness of 30 mm or more, and it is believed that this leads to a decrease in impact value and to spring breakage. It is known that higher spring stress increases sensitivity to hydrogen embrittlement cracking and the fatigue strength at which pitting occurs in a corrosive environment.
There are various types of steel in which hydrogen embrittlement resistance is increased through an increase in the fatigue life of spring steel (see Japanese Patent Publication 2001-234277, for instance), but no steel has yet to be developed that combines high stress with high toughness as in the present invention.
The present invention was conceived in light of the above prior art, and provides a spring steel that has superior hardenability, undergoes less pitting in a corrosive environment, and has a higher strength and toughness, even in large-diameter suspension springs with a diameter of 30 mm or more and thick leaf springs with a thickness of 30 mm or more.